The outcome of the AV referendum will be decided by Labour votes. The most recent YouGov poll, for instance, shows that while Lib Dem voters are overwhelmingly in favour of reform (79:13) and Conservative voters are overwhelmingly opposed (66:20), Labour voters are split 40:39 in favour of first-past-the-post.
With this in mind, it's worth watching to see how Labour activists respond to a new YouGov poll for Channel 4 News showing that their party would suffer the most from a switch to AV. Under FPTP, based on current voting intentions, Labour would win 355 seats, the Tories would win 255 and the Lib Dems would win just 16 – a Labour majority of 60. But under AV, Labour would win 342 (-13), the Tories would win 255 (unchanged) and the Lib Dems would win 29 (+13), resulting in a significantly reduced Labour majority of 34.
At every general election from 1997-2010, Labour would have done better under AV thanks to a high number of second-preference votes from Lib Dem supporters. But, as previous polls have shown, Lib Dem voters now split in favour of the Tories (a large number of their left-wing supporters having already defected to Labour).
The latest poll shows that 31 per cent would back the Tories, 24 per cent would back Labour and 24 per cent would back the Greens. Returning the compliment, 41 per cent of Conservatives voters would give their second preferences to the Lib Dems, followed by Ukip (27 per cent).
Unsurprisingly, Labour support for the Lib Dems has collapsed since the election, with even Ukip preferred to Nick Clegg's party.
A May 2010 poll by the British Election Study showed that more than half of Labour voters would give their second preferences to the Lib Dems, but now just 16 per cent would. The Greens (30 per cent) and Ukip (18 per cent) both attract more support than the yellow team.
As YouGov's Anthony Wells points out, the poll comes with several health warnings: "[I]t assumes both a uniform swing, and that each party's second preferences split in the same proportions across the country. It also cannot take into account what effect an election campaign fought under AV would be."
But, for those in Labour struggling to explain the merits of AV to an increasingly tribal and parochial party, this is another blow.